Fix style constraint save when strength is NotSet

[elpaso]

Fix #58431

I feel that the underlying issue remains (which is that setting the strength to NotSet removes it from the field but not from the layer and that the default strength is Hard) but since it was clearly made in purpose 8 years ago I am not keen to change it lightly.

[elpaso]

@nyalldawson this fixes the issue #58431 but not the root cause and I'd like an opinion about the possibility to change
https://github.com/qgis/QGIS/blob/master/src/core/qgsfieldconstraints.cpp#L32
(see also: https://github.com/qgis/QGIS/blob/master/src/core/qgsfieldconstraints.cpp#L40) to be consistent with QgsVectorLayer::setFieldConstraint which doesn't remove the constraint if strength is NotSet.

The current behavior is not consistent.

[github-actions]

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[nyalldawson]

@elpaso

default strength is Hard

It's too long ago for me to remember exactly, but I suspect this was done in order to preserve API (ie before the introducing of soft constraints, all constraints where implicitly "hard" but we weren't explicitly tracking that).

I'm happy for you to tweak this logic as needed -- there's good test coverage there, so I'm confident we'll be alerted if you break anything in the process..